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Community and Q&A

Is 3″ thickness really enough for a basement slab?

user-884737 | Posted in GBA Pro Help on

Hallelujah! 1″ XPS board is down, so is the StegoWrap vapor barrier, the rebar and soon the 6″x6″ wire mesh. So the new basement slab in the old wet basement will be poured on Monday. It already smells better!! (Might have something to do with discovering and fixing the broken sewer pipe just outside the foundation, that was draining back into the gravel basement floor – yuck!!). Today I reviewed the diagram on “How to Finish Exterior Foundation Insulation” and it shows a slab thickness of 3″. My concrete guy insists on 4″; however, to make it all level it’s really going to end up being 5″ in places. Unfortunately that means that the new floor will be an inch higher than the one in the connecting room.
Really, a 3″ thick slab? That would solve my mismatched floor levels. BUT… what about:
…The inevitably imperfect stone levels underneath, detected when walking on the blueboard;
…The drainage tile network (and I mean network) underneath, bedded in the stones (yes, surrounded by stones and filter cloth burrito) potentially causing flex due to their hollowness.
Seems to me the rebar network (set on plastic chairs at 2 1/4″; cross bars atop that adds another 1/2 more height) topped by 6×6 mesh could offer plenty of stability/strength but all that stuff needs enough concrete around it. Hence the 4″+.

How can the 3″ in the diagram be enough??

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  1. Riversong | | #1


    3" of concrete is more than enough for a slab that is only a finished floor and not a load-bearing member, particularly with that excessive amount of steel reinforcement (typically it's either rebar or mesh but not both).

    I would insist that the contractor use shorter chairs and minimum 3500 psi concrete with fiber reinforcement and keep the new floor at the same height as existing.

  2. David Meiland | | #2

    I guess I'd grudgingly agree to let the slab taper to 3" in places, but I would definitely lower the rebar mat so that the bottom of the bottom bar was 1" off the XPS. It should be easy to buy 1" dobies (they're little blocks on concrete with tie wires sticking out) or in a pinch use small chunks of broken flagstone or something else that's the right thickness.

    I would ask for a 6-1/2 sack mix batched for a 3" slump with water reducer added to produce a 7" slump. The batch plant I buy from sends out a great slab mix using those instructions. I always use fiber and never use wire mesh.

    The wet concrete will flatten the XPS down against the gravel base unless there are big dips. If it was leveled carefully with a grade rake you should be good.

  3. David Meiland | | #3

    Dobies... first column third row.

    Not to be confused with the dog breed.

  4. Riversong | | #4

    You can also get plastic rebar chairs as short as 1" and wire chairs as short as 3/4".

  5. user-884737 | | #5

    Thanks so much for your input!! You helped me decide to remove those 2 1/4" plastic chairs, since they're too high. Couldn't find shorter ones that would work over the vapor barrier in the 2-day, over-the-weekend time frame (wire feet are a no-no; no time to order those cool little dobies) . So, we used little blocks of 1" blueboard (XPS) with little home-made wire staples (not too long! about 1 1/8") pushed down over the 1/2" rebar into the block, to hold it there. Then bricks set under the middle of the wire mesh panel. Seemed to help the rebar and the mesh bounce back up after being walked all over. Good news is they were able to reduce the total height of the slab and now the transition to the existing slab works great! Yay! The concrete pouring guy said he uses XPS foam board pieces too as rebar lifters/chairs - only he uses 2" thick XPS. It was simple to make the spacers, too - I had leftover 4" wide pieces (side-of-slab insulators) and just scored and broke those into 2"x4"x1" sections.
    Well, here's hoping that passes muster with you folks - if not then you could pass on your remedial advice to the next reader and consider our approach as permanently "entombed"!
    Thanks again. Your rapid responses have been very helpful and very much appreciated!

  6. Riversong | | #6

    Yay Anita. You did good. Most homeowners just end up getting bowled over by the "experts" who insist on doing it their way.

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